by Much of the US will soon be hit by another Arctic blast, adding to the already cold January 2025. Forecasters expect temperatures to fall below zero in several states, dropping to above minus 30 degrees in parts of the Dakotas, Minnesota and Wisconsin. In addition to the cold polar air, further winter storms are expected.
Meteorologists warn that temperatures in some areas could be 30 degrees or more “below normal” at times between January 18 and 22.
But what does “normal” actually mean?
While temperature forecasts are important to keep people safe, comparing them to “normal” temperatures can be quite misleading. Because what is considered normal in the forecasts has changed rapidly over the years as the planet warms.
Define normal
One of the most commonly used standards for defining a science-based “normal” is a 30-year average of temperature and precipitation. Every 10 years, the National Center for Environmental Information updates these “normalities,” most recently in 2021. The current period considered “normal” is 1991-2020. Five years ago it was 1981-2010.
But temperatures have been rising over the last century, and the trend has accelerated since about 1980. This warming is driven by the mining and burning of fossil fuels, which leads to increases in carbon dioxide and methane in the atmosphere. These greenhouse gases trap heat near the planet’s surface, causing temperatures to rise.
As global temperatures warm, warming is also considered normal.
So when a cold snap in 2025 is reported as the difference between actual and “normal” temperatures, it appears colder and more extreme than when compared to a previous 30-year average.
Thirty years is a significant part of a human life. For those under approximately 40 years of age, using the most recent average range may suit their experience.
But it says nothing about how much the Earth has warmed.
What cold snaps are like today compared to before
To see how today’s cold snaps – or warming – compare to a time before global warming began to accelerate, NASA scientists use the years 1951-1980 as a baseline.
The reason becomes clear when you compare cards.
For example, January 1994 was brutally cold east of the Rocky Mountains. If we compare these 1994 temperatures to today’s “normal” temperatures – the period 1991-2020 – the U.S. resembles maps of temperatures in early January 2025: Large parts of the Midwest and Eastern U.S. had more than 7 degrees Fahrenheit (4 degrees Celsius). Celsius). below “normal” and some areas were much colder.
However, if we instead compare January 1994 to the 1951-1980 baseline, this cold patch in the eastern United States is not quite as large or extreme.
While temperatures in some parts of the country in January 1994 were 14.2 F (7.9 °C) colder than normal compared to the 1991–2020 average, they were only close to 12.4 F (6.9 °C ) colder than the 1951–1980 average.
As a measure of a changing climate, updating the 30-year average baseline every decade makes warming appear less than it is and cold snaps appear more extreme.
Charts show how temperatures have changed in southwest Minnesota. Each histogram on the left shows 30 years of average January temperatures. Blue is the most recent 30-year period, 1991-2020; Yellow is the earlier period 1951-1980. The bell curves of the frequency of these temperatures show a shift of about 4 F (2.2 °C). Omar Gates/GLISA, University of Michigan
Conditions for heavy lake effect snow
The U.S. will continue to experience cold air outbreaks during the winter, but as the Arctic and the rest of the planet warms, the frigid temperatures of the past will become less common.
This warming trend is contributing to the remarkable situation in the Great Lakes that we are experiencing in January 2025: heavy lake-effect snow over a large area.
When cold Arctic air moved in from the north in January, it hit a Great Lakes basin where water temperatures were still above 4.4°C (40°F) in many places. On January 4, ice covered less than 2% of the lake’s surface. Even after two weeks of cold air, ice cover remained at less than 10% on January 14th.
This cold, dry air over warmer open water causes evaporation and provides moisture for lake-effect snow. Heavy snow fell in parts of New York and Ohio along the lakes.
The heat build-up observed year after year in the Great Lakes leads to fundamental changes in winter weather and the winter economy in the states bordering the lakes.
It is also a reminder of the ongoing and growing presence of global warming, even in the midst of a cold air outbreak.
This article has been updated with details about the recent explosion in the Arctic.
This article was republished from The Conversation, a nonprofit, independent news organization that brings you facts and trusted analysis to help you understand our complex world. It was written by: Richard B. (Ricky) Rood, University of Michigan
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Richard B. (Ricky) Rood receives funding from the National Oceanic and Atmospheric Administration.
